Texture not realistic
Ok... As I am a new user, and have insufficient reputation points to post this as a comment (which I think is what it should be) I am reframing it as a PARTIAL answer, as the meta.stackexchange.com post suggests I should: (see Why do I need 50 reputation to comment? What can I do instead? )

I wanted to add to what Duarte Farrajota Ramos said about surface tension. Remember the surface tension and the bubbles make the liquid surface non-flat, so you need the surface normals to form a dome over bubbles and to creep up around the edge of the marshmallows and cup to look real... Also good to think how the bubbles interact with each other, and with the marshmallows: The bubbles should clump togeather and eventually merge or pop, they also stick to the sides of the marshmallows and cup, but they can't occupy the same space as each other nor intersect the edges of the cup or marshmallows. You may want to look at Jean-Francois Gallant (pyroevil)'s "Molecular Script" blender addon to allow particles, that you may use for bubbles, liquid, and marshmallows, to interact with each other. (Standard Blender particles can interact with their environment, but not each other.)